The present section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Low-Dynamic-Range images (LDR images) are images whose luminance values are represented with a limited number of bits (most often 8 or 10). This limited representation does not allow correct rendering of small signal variations, in particular in dark and bright luminance ranges. In high-dynamic range images (HDR images), the signal representation is extended in order to maintain a high accuracy of the signal over its entire range. In HDR images, pixel values are usually represented in floating-point format (either 32-bit or 16-bit for each component, namely float or half-float), the most popular format being openEXR half-float format (16-bit per RGB component, i.e. 48 bits per pixel) or in integers with a long representation, typically at least 16 bits.
The problem to be solved by the invention is to provide an automatic or semi-automatic method for tone-mapping a HDR image into a LDR image or, by extension, a sequence of HDR images into a sequence of LDR images because the method is applied image per image.
Tone-mapping methods are useful, for example, to view a HDR image on a traditional LDR-compatible display or in a dual color-grading approach which performs usually in a broadcast scenario.
Typically, a dual color-grading approach comprises capturing a HDR image (or sequence of images), color-grading the captured HDR image and tone-mapping the HDR image into a LDR image which is then color-graded.
As a matter of fact, because a tone-mapped LDR image is intended to be display, it must fulfil the basics of a viewable LDR scene compliant with the underlined HDR scene. In other words, the tone-mapping must preserve at least the spatial coherence into the tone-mapped image, the temporal coherence between multiple tone-mapped images of a sequence of images, the global luminance coherence, i.e. dark HDR scenes lead to dark LDR scenes and vice-versa, and the color coherence between an HDR image and a tone-mapped image i.e. the color-grading is preserved as much as possible.
A dual color-grading approach leads to the highest quality for a LDR image as it is produced under the control of an expert in color-grading, thus ensuring the absence of uncontrolled undesired effect of automatic and systematic methods. However, it takes the double post-production resources as one has to deal with two workflows, one for HDR and one for LDR.
It may not be practical for all use cases and it may the case that only the HDR sequence has been color-graded. Of course, in such a case, it is desirable to obtain a viewable LDR image because not all customers may be equipped with HDR devices for displaying. In order to reach these customers, a LDR image must be produced without extra or alternative color-grading, i.e. by using an automatic tone-mapping method providing LDR images with an acceptable quality for all contents.
Many local or global tone-mapping operators exist in the prior art such as, for example, the tone-mapping operator defined by Reinhard may be used (Reinhard, E., Stark, M., Shirley, P., and Ferwerda, J., Photographic tone reproduction for digital images,” ACM Transactions on Graphics 21 (July 2002)), or Boitard, R., Bouatouch, K., Cozot, R., Thoreau, D., & Gruson, A. (2012). Temporal coherency for video tone mapping. In A. M. J. van Eijk, C. C. Davis, S. M. Hammel, & A. K. Majumdar (Eds.), Proc. SPIE 8499, Applications of Digital Image Processing (p. 84990D-84990D-10)).
However, even if global tone-mapping operators preserve temporal properties over a sequence of images such as the brightness consistency, global tone-mapping operators may fail dramatically when applied to sequences of HDR images with a lot of dynamic as such operators are unable to represent the spatial details on a lower dynamic without a noticeable loss of sharpness.
On the other hand, local tone-mapping operators are not efficient when dealing with sequences of images because they are based on local pixel neighborhood to capture local properties of an image and do not preserve the overall brightness of a sequence of images (temporal brightness consistency). As a consequence, temporal brightness consistency is obtained by using additional complex processing such as object tracking or frame buffering in order to apply temporal filtering.